Testing of Vacuum Cb

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TESTING OF VACUUM CIRCUIT BREAKERS: SPECIFIC ISSUES AND DEVELOPMENTS

R.P.P. Smeets, A.G.A. Lathouwers, L.H. te Paske

KEMA High-Power Laboratory, the Netherlands

ABSTRACT

In the international IEC standards for medium and high- voltage circuit breakers no distinction in test requirements is made between circuit breakers with different arc-quenching media, like SF6, oil and vacuum. This had led to the situation that due to the different technology of arc interruption applied, various aspects of the prescribed test-procedures have a different weight in terms of severity for various types of breakers. Add- itional test-requirements and interpretations that are specific to vacuum circuit breakers (VCB) have been for- mulated commonly by the joint major test laboratories. In this contribution, the background and practical application of several procedures, now generally adopted by certifying test-laboratories, regarding the peculiarities of vacuum circuit breakers are elucidated. Because the vacuum circuit breaker has an excellent capability to interrupt current of high-frequency, the main part of this contribution will focuss on the consequences for test procedures of this aspect, not encountered in SF6- or oil breakers. In particular, the judgement of non- sustained disruptive discharges, multiple reignition and virtual current chopping in test-circuits is addressed. In addition, results of a new high-resolution high-frequency current-zero measuring system are presented. This system is able to give insight into the high-frequency arc phenomena in the immediate vicinity of arc interruption, and is designed to get more specific information on arc behaviour in standard high-power tests.

TESTING OF VACUUM CIRCUIT BREAKERS

Vacuum, as an arc interrupting medium, is dominating the distribution market up to a voltage of 36/38 kV. This is reflected in the number of certificates for this technology issued in this range by =MA, worlds largest high-power laboratory. In fig, 1 an overview is given of the number of certificates issued by KEMA from 1991 - 1996 (according to IEC and ANSI) on switchgear with a differentiation in the rated voltage and the arc quenching medium. The rated voltage is choosen from 3.6 - 145 kV, covering the scope of this symposium. It is clear, that above 38 kV, SF6 is the dominant medium, although vacuum now starts to penetrate in the upper medium voltage. In the voltage range 3.6 - 145 kV, 450 certificates were issued, 57% for vacuum equipment (mostly circuit breakers), 29% for SF6 switchgear (mostly circuit breakers) and 14% for air- and oil based switchgear (disconnectors and earthing switches).

3.64.76 5 7.2 12 1515.517.524 2525.327 36 3872.5100123126145 rated voltage (kV)

Figure 1: KEMA certificates on switchgear 3 - 145 kV

These numbers indicate that for the distribution voltages vacuum is a mature interruption technology, deserving careful attention from standardizing bodies and testing laboratories in order to assess the peculiarities of vacuum - both in a positive as well as in a negative sense - in a proper way. For circuit breakers, the standard IEC 56 has found the widest acceptance (applied for in 92% of the circuit breakers offered to KEMA for certification from 1991 - 1996). It is the philosophy of IEC, not to let the test- requirements depend on the technology applied in the circuit breaker. This is acceptable from the point of view that circuit breakers should be completely versatile in their application. On the other hand, the various peculiarities, inherent to the interruption medium, can not justify a single uniform set of test-requirements for all technologies of breakers. Since from historical reasons, the IEC 56 requirements are based on the behaviour of SFs and oil circuit breakers, some requirements in the standard are less relevant for vacuum. An example of this is the requirement of demonstrating the interruption capability of low values of short-circuit current (down to 10% of the rated value) at increased (up to 5 times) rate of rise of transient recovery voltage, notably occurring at transformer secondary faults. This stems clearly from the difficulty SF6 can experience under high rates of rise of TRV due to the inherent thermodynamic properties of the gas,

Trends in Distribution Switchgear, 10-12 November 1998, Conference Publication No. 459 0 IEE 1998

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whereas vacuum can cope with such a stress normally without any difficulty (1). Other test-procedures, also coming from the SF6 tradi- tion, simply can not be fulfilled for vacuum interrupters (neither for other sealed-for-life devices) as required, such as the visual inspection of the contacts to assess the state of the breaker after short-circuit tests. On the other hand, some specific vacuum-related phenomena, originating from observations during the two decades of experience in service, are not properly covered by IEC 56, and need additional attention. To this aim, STL (Short circuit Test Liaison), an international organisation in which the major test-laboratories co-operate, is constantly active to issue additional procedures and interpretations for situations that are insufficiently or not unambiguously covered by IEC (2).

NON-SUSTAINED DISRUPTIVE DISCHARGES

Non-sustained disruptive discharges (NSDD) are a possible concern in the application of vacuum switchgear. This phenomenon causes a temporary bridging of the vacuum insulation relatively long (up to seconds) after the interruption of short-circuit current. The breakdown is thought to be initiated by particles, loosened by mechanical vibrations. The phenomenon is associated with a high-frequency (HF) current due to the discharge of local stray capacitances. Because vacuum can interrupt the high frequency current, the duration of the discharge is very short (few p), and - depending on the circuit - does not necessarily lead to re-establishment of (a single loop) of the fault current just interrupted. Since this is a typical phenomenon inherent to vacuum switchgear only, no specific mentioning is made in the relevant standards (eg. IEC 56) that cover all types of circuit breakers. This situation has stimulated STL to issue the additional requirement to examine the occurrence of NSDD during type testing of VCBs. To this end, it is required that VCBs after short-circuit current interruption are subjected to a maintained power frequency (rated) voltage for at least 300 ms after interruption (in contrast to 100 ms for other breakers). If there are more than three oc- currences of NSDD during the entire series of test- duties, certification will not be possible. Restoration of 50 Hz current resulting from an NSDD is never allowed.

The question arises next, how to design proper and at the same time realistic test-conditions that allow an unambiguous detection of these very short HF phenomena. This is not trivial, since the frequency band in which the NSDD phenomena occur is beyond that of standard phenomena associated with short-circuit testing. This means, derived features must be used to observe the occurrence of NSDD.

A typical three-phase test-circuit is sketched in fig. 2. In principle, grounding can be at point S (source neutral), point T (TRV circuit), point N (neutral) and all combi- nations. At the moment of breakdown of the vacuum gap, a

L, VCB L p -rvvv\

1 T cs

1 TCN

Figure 2: Generalized lay-out of test circuit

variety of currents (may) start to flow. The larger the contributing circuit parts are, the lower the frequency of the current is:

I .VHF current: Independent from grounding, the initial current at breakdown is always supplied by thepara- sitic elements in the immediate vicinity of the interrupter. This current is of a very high frequency (VHF, > 10 MHz), and is not interrupted.

2. HF current: A current of high frequency (10 kHz - 1 MHz). Depending on the grounding, two situations must be distinguished:

T&N grounded (HFlow current) a relatively high (up to several kA) current arises at a relatively low frequency (in the order of 10 - 100 kHz). This current is determined by the TRV capacitors C, and the stray inductance L,.

all other cases: (HFhigh current} a relatively low current of high-frequency (order MHz) flows, determined by the stray elements CT, CN, L ,

Since the di/dt of the HF current at current-zero has the same order of magnitude (hundreds of Alps, this quantity is only depending on the stray inductance Lp), the probability of interruption of the HF current is roughly independent of the circuit topology.

3.LF current. Power frequency (short-circuit) current can now start to flow, provided the VCB does not interrupt the HF current. In such a case, the discharge is no longer an NSDD, and will not be treated as such.

TABLE 1 - Components of NSDD-initiated current at various grounding conditions of the circuit

I # I ground I VHF I HFhigh I HFlow I LF I

no yes yes

Table 1 summarizes the different components of current that (can) flow following breakdown. For the possibility of detecting NSDD, the grounding is

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an essential parameter. During testing, high-frequency current in the circuit breaker is normally not measured with a bandwidth matching the high-frequency of the phenomenon. Therefore, the voltage at the source side of the breaker must be used as a means to detect NSDD. 1 case 3, 5: Here the relatively low frequency of current

allows an unambiguous detection; at the moment of the discharze, the voltage over the breaker collapses, leaving a relatively long voltage-zero period during the conduction of the HF current. Even in the case of a small number of loops of NSDD current, voltage measuring systems have sufficient bandwidth to detect the NSDD.

1 case I , 2, 4: Now, the situation is different, since after interruption of a single loop of NSDD current, the voItage is almost immediately (within a few ps) re- stored to the value prior to the NSDD, but ofoppo- site polarity, and can thus easily be detected thanks to the stepwise voltage change, even when insuffic- ient bandwidth of the voltage measuring system is available. This is also the case at any odd number of HF current loops. Care must be taken with the situation in which an even number of HF current loops passes the breaker, since in this case only a very short (few ps) interval of voltage zero occurs followed by an immediate restoration (of the same polarity) of voltage after this period. For this case, voltage measuring systems having a sufficient bandwidth are essential.

a case 4, 5: This is never a problem, since restart of power frequency current is always unambiguous. This situation is then no longer called an NSDD, since the discharge is sustained now. This is the situation in single phase testing.

I

I

odd hf loop; even hf

Figure 3: Schematic NSDD current- and voltage

In fig. 3, the schematic voltage and current waveshapes for the two situations HFlow and HFhigh are illustrated. From the argumentation above, it is clear that the meth- od of grounding of the test-circuit and the bandwidth of the voltage measuring system is essential in the detection of NSDDs.

At high-current testing at KEMA, grounding is normally as in case 1 (point N in fig. 2). This is because IEC 56 is based on a threephase-to-ground fault. Whether this always covers the practical situation, could be subject to futher study. It is KEMAs experience that - although NSDD is most frequently observed after breaking of short-circuit current - the occurrence at reduced current can not be ex- cluded. Even at capacitive switching current (few hundreds of ampkres) late breakdowns were recorded, but in this case they are treated and registered as restrikes. A measured example of NSDD (83 ms after breaking 32 kA of a 15 kV VCB) in phase 3 is shown in fig. 4.

_ _ - .. . . . . I I current phase 1

NSDD I

I I .. I voltage phase2 I I

i \ -v currentphase3 Figure. 4: Measured example of NSDD

OVERVOLTAGES IN TEST-CIRCUITS

For interruption duties, where there exists a strong interaction between arc and circuit, test-circuits must be designed with ample knowledge of the relevant phenomena. Such a duty is the interruption of small inductive current, which needs attention, especially for VCB. A phenomenon, sometimes observed under very specific conditions in the application of VCB, not known with other type of breakers, is virtual current chopping. Virtual current chopping is the forced arc extinction in a phase of the VCB far from current zero. Such a forced current-zero is induced by HF current that arises due to a reignitionhestrike in a neighbouring phase. The prob-

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ability of occurrence of virtual current chopping is very much depending on the circuit, especially on the mutual inductance andlor capacitance of the circuit parts where the HF reignition current flow (3). This has consequences for the choice of test circuits for tests with small inductive currents, especially its HF characteristics.

71

LV

1

..e

Figure 5 : Example of multiple reignition and virtual current chopping of VCB. 8 kVldiv, 0.6 msldiv

has been acknowledged by IEC in the prescription of the detailed high-frequency character of test-circuits (IEC 1233) and the requirement that also the value of the maximum peak-to-peak voltage (U,) at the motor terminals at each test must be evaluated in the test-report. In all other cases, where the details of the small inductive test-circuit are not or can not be prescribed, values of peak overvoltages are of limited use in assessing the overvoltage generating tendency of vacuum interrupters.

TOPOLOGY OF TRV CIRCUITS

In circuits for short-circuit current interruption tests, the waveshape for the transient recovery voltage (TRV) can in principal be produced with a series damped or parallel damped circuit, see fig. 6. The principal difference between the two is the rate of rise of voltage immediately after interruption.

l.M I I

0 00 timE(ut) 120 160 200

A typical example is shown in fig. 5 , where a 38 kV VCB was tested with a current of 20 A, supplied by a

Figure 6: Series- and parallel damped TRV and circuit with equal values of the elements.

circuit simulating the interruption of magnetizing current. In this figure, virtual current chopping in the lower phase results from the first reignition in the upper phase. Overvoltages caused by virtual chopping are inherently limited (here) by the (short) gaps of these phases. Values that characterize the interruption - and that are entered in the test-report - are the suppression peak voltage U,, (this voltage is directly related to the current chopping of the VCB) and the voltage across the breaker at reignition U, (see fig. 5) . Since the layout and topology of the test-circuit deter- mines both the probability of producing such overvoltages and the magnitude of the overvoltages, circuits for these small inductive current interruption tests must be designed with the utmost care in order not to allow the VCB under test produce more severe overvoltages than it should do in a specific application. This is a genuine hazard, since test-circuits tend to be more compact than real circuits, thus increasing the risk of introducing HF current in unexpected paths. Generally valid statements on maximum overvoltages produced by VCB based on tests therefore are impossible, and the measured values of the overvoltages are valid in the test-circuit only.

For the specific case of switching motors, this situation

In test-laboratories, the parallel damped circuit must be realized with lumped elements the capacitance of which is located close to the test-breakers, in order to prevent any unwanted initial rise of the TRV. For the parallel damped circuit, the resistor bank dissipates a consider- able amount of energy. A hybrid solution is a series damped circuit with a (delay) capacitance in parallel, resulting in a zero initial rate of rise. At a voltage breakdown at any moment following interruption, the resulting HF current will initially be supplied by the TRV circuit, causing a different waveshape depending on what circuit is used. In the parallel damped circuit, a high-frequency (periodical) current will flow, that may be interrupted by the VCB, causing a successful interruption possibly associated with overvoltages. In the series damped case, the reignition current can have an a-periodical waveshape, allowing the 50 Hz current to re-establish and the interruption to fail. This principal is outlined in fig. 7. Therefore, the choice of the TRV circuit topology, has consequences for the interruption process, though the (inherent) TRV waveshapes can be identical. It is KEMAs practice to use the parallel damped circuit for VCB testing, since field tests have shown an oscil-

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latory reignition current to be a more realistic option (4).

I ,. I 1.1 periodical reignition current (parallel damped)

interruption 01 HF reignition current I \ i I

Figure 7: Waveshape of reignition current in series - and parallel damped TRV circuits

CONTACT PHENOMENA

Since a vacuum interrupter cannot be opened for contact inspection, as required in the short-circuit current interruption standard IEC 56, other means were agreed upon within STL to assess the condition of the contacts after short circuit testing. To this aim, the contact resistance of each interrupter is taken as a measure, in a compari- sion before (value provided by the manufacturer) and after short-circuit tests (value measured by the test laboratory). If the resistance is not increased by more than 200%, this part of the inspection is satisfactory. In order to verify the insulation integrity after short- circuit current tests, a one minute power frequency voltage at a level of 80% of the rated withstand voltage (or at least 110% of the full open contact breakdown voltage for that vacuum interrupter with atmospheric air pressure - whichever value is the highest) is applied. These procedures are not inherent to vacuum, but they are applied for all so-called sealed-for-life breakers.

Contact welding, being specific to VCB due to the primitive butt-type contacts, can occur due to making operations of short-circuit current or as a result of the energizing of capacitor banks, especially in the case where other capacitor(s) supply a large inrush current in the capacitor to be energized (back-to-back switching). Under these conditions, significant thermal arc energy is dissipated between the closing contacts from the moment of the breakdown of the gap - the prestrike - until the end of the contact bouncing period. In this situation contact welding of the contacts may occur. Usually, such a weld is broken by the mechanism at the next opening operation, but the remaining (micro) protrusions on both contacts can degrade the insulation integrity, especially in those cases where the making operating is followed by a breaking operation with an arc-current too small to remelt the protrusions. Such a case is the back-to-back switching with small capacitive current. In this duty, the revised edition of IEC 56 will required 20 kApeak at 4.2 kHz to be switched in, fol-

lowed by the interruption of only 400 A of capacitive current. Contact welding can also occur as a result of so-called Short Time Current (STC) tests. In such a test, high current is passed through the closed contacts during a time of 1- 3 s. Due to the butt-shape of vacuum contacts, they tend to separate slightly under high-current passage, promoting welding. Sufficient mechanical contact force must counteract this. It is the experience that a STC tests after short-circuit tests is more severe than in virgin condition of the contacts, due to the increased contact resistance by erosion. In the present IEC standard, no order of testing is required, so from the manufacturers point of view to start with STC-test on a virgin VCB has preference, whereas from the user point of view the VCB must be able to withstand an STC also after high-current interruption.

A NEW CURRENT-ZERO MEASURING SYSTEM

Innovative testing laboratories are constantly develop- ing additional services in order to give their customers more information on the test results than the answer to the question whether or not a test-object has passed the tests. Therefore, new methods to draw additional information during standard tests, helpful for designer and applicant are developed. Essential information on the margins of interruption can be acquired during the short interval around current zero. For vacuum, the post-arc current (i.e. the current immediately after interruption) gives information on the decay rate of the charged particles still present in the contact gap the first few ps after current interruption.

The development of current zero measurements has a long history, also at KEMA ( 5 ) where in an early phase, the choice was made for a Rogowski-coil based system. To this aim, a special Rogowski-coil was designed, with a bandwidth up to 10 MHz (4). The main problem in current zero measurements has always been the great dynamical range that the current measuring system must be able to cope with: tens of kA before current zero and less than 1 A (the post-arc current) immediately after current interruption. Within the framework of a project carried out with an international consortium and sponsored by the European Commission (contract nr. SMT 4-CT96-2121), a current zero measuring system for the highest voltages and currents was designed and constructed by KEMA (6). Three single channel A/D converters (located near the test object) with a sampling frequency of up to 40 MHz and a 12 bit resolution store the data in a local RAM memory of 256k samples. The total memory can be seg- mented in up to four parts, in order to capture multiple current-zero windows during a single high-power test. Immediately after a test, the temporarily stored data are automatically transmitted over the fibre to a back-up memory in the central control unit, located in the control room of the laboratory. Via an IEEE bus, access to the data from a PC for further data processing is obtained. In order to exploit the possibility of potential-free

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measurement, the digitizers are operating fully auto- nomous from the outside world. The connection with the central control unit is through optical fibres (60 m) taking care of triggering, remote control of sampling frequency, sensitivity etc. The other two digitizers re- cord the voltage over the circuit breaker, in order to cor- relate (arc) voltage with current. An outline of the system is given in fig. 8. Apart from problems, inherent to the processing of dis- cretisized signals from Rogowski coils, a number of corrections are necessary which are inherent to the parasitic components of the test-objects and -circuit. This has resulted in a software package that offers pos- sibilities for viewing, zooming, measuring and trans- forming of the measured data based on MS Windows 95 concepts. As part of the acceptance tests, EMC tests were done with 100 kAPeak and voltages up 10 kV/ps (600 kVPak).

A result of a current-zero measurement is shown in fig. 9 giving a typical test of a VCB (current was 30 ARMS) subjected to a high-frequency (300 kHz) TRV. This figure shows a post-arc current of 800 mA (peak) with a duration of approx. 2 ps. A first impression of the dy- namic sensitivity of the system is obtained when observ- ing in fig. 9 the small capacitive current (approx. 50 mA) that is drawn through the parasitic capacitance (few tens of pF) of a circuit breaker under the influence of a high-frequency TRV.

I y l t a g e div ider I 1 -- &

unit -U Figure 8: Sketch of the measuring system

SUMMARY AND CONCLUSIONS

Vacuum switchgear is dominant now on distribution le- vels thanks to its excellent properties regarding current interruption and freedom of maintenance. Nevertheless, a few minor aspects of typical VCB behaviour, relevant for type-testing and certification need attention:

The problem of the detection of non-sustained disruptive discharges during high-current test and the influence on the test circuit-topology. Their occurrence is monitored up to 300 ms after interruption. Up to three NSDDs are allowed for all the tests in one program. . The importance of the interaction of VCB with the (parasitic) circuit during reignition regarding the gene-

ration of switching overvoltages. Overvoltages in test- circuits are not representative for reality, except in the well defined case of motor-switching.

m The necessity of a proper choice of the TRV wave- shaping circuit for VCBs. Interruption capability in testing can differ with the same (inherent) TRV. . Problems related to the specific contact shape of VCBs for testing regarding contact welding. Increase of contact resistance and deterioration of dielectric withstand occurs more easily in VCB than in gas circuit breakers.

A new current-zero measuring system is introduced, being capable to measure post-arc current downto a level of 50 mA after short-circuit current interruption.

\ I 0.00 c

i current I I lime (us) I

2 4 6 8 1( -1 .oo

-4 -2

Figure 9: Current-zero measurement after 30 kA VCB interruption with high-frequency TRV

REFERENCES

(1) Smith R.K., 1994, Tests show ability of vacuum circuit breaker to interrupt fast transient recovery voltage rates of rise of transformer secondary faults, = Trans. Pow. Delivery, Vol. 10, No. 1,266-271 (2) Guide to the interpretation of IEC Publ. 56, STL, 1988, STL, Rugby, England (3) Smeets R.P.P., Lathouwers A.G.A., 1996, Switch- ing Surges Associated With Vacuum Interrupters In Motor Circuits, 1lth Conf. on Elec. Pow. Supplv Ind. fCEPSI), Kuala Lumpur (4) Damstra C.G., Hooijmans J.A.A.N., 1990, Influen- ce of TRV Network on Circuit Breaker Interruption Performance at Terminal Fault Conditions, CIRED Conference ( 5 ) Damstra C.G., Kertesz V., March 1995, Develop- ment and application of a 10 MHz digital system for current-zero measurements, IEE Proc.-Sci. Meas. --- Technol., Vol. 142, No. 2, 125 - 132 (6) Smeets R.P.P., Even A., Habedank U., Kertksz V., Neumann C., Scarpa P., van der Sluis L., 1998, Pro- gress towards Digital Testing, a novel additional tool to investigate the performance of HV circuit breakers for the benefit of utility, manufacturer and standardizing body, CIGRE Conference.

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Testing of Vacuum Cb